This invention relates to polymer particles comprising monomer units of at least one reactive mesogen, a process for their preparation, the use of these particles for the preparation of optical, electrooptical, electronic electrochemical, electrophotographic, electrowetting and electrophoretic displays and/or devices and security, cosmetic, decorative, and diagnostic applications, and electrophoretic fluids and displays.
Reactive mesogens (RMs), when polymerised at temperatures at which they exhibit thermotropic liquid crystal phases (typically nematic, cholesteric or smectic), produce polymers with a degree of optical anisotropy. This property has been widely exploited in the field of optical films for compensation and brightness enhancement in flat panel displays, especially liquid crystal displays.
It has been disclosed in WO 2003027761, DE 19602848, DE 19602795, JP 2001262144 and WO 2004005425 that a range of RMs can be used to prepare particles (sometimes described as flakes). These particles can reflect colours of various wavelengths and have proven applications in pigments, and broadband reflection films. In addition, a method of using particles containing various amount of RMs in electro-optical devices is disclosed in JP 10062739 which discusses a thermally switching PDLC device.
In 2001, Crawford et al. (Applied Physics Letters, 78(18), (2001), 2643-2645) prepared particles from RM257 using an emulsification process in glycerol, followed by an in-situ photopolymerisation step with UV light. These particles rotate under in-plane electric fields. Shafran et at. (Mater. Res. Soc. Symp. Proc. 1096 (2008)) also reported nano-rods made from RM257 using a template and UV light to fix the conformation. These rods show rotational and translational movement under DC and AC electric fields.
Preparation of anisotropic LC particles has been also described by Fernandez et al. (Soft Matter, 2, (2006), 105-108) using microfluidics to produce monodisperse particles in water. By increasing the quantity of PVA it is possible to make a film that can be stretched to obtain tactoidal particles. This conformation can be fixed by polymerization of the stretched film under UV light. Although this process can be used to obtain particles that have both shape and internal anisotropy, it is not a synthetic method that is suitable for making particles that can be directly used in an electro-optical device because the solvent used is too polar.
Sandomirski et al. prepared 100 nm-1 μm colloidal particles of RM257 by emulsification in aqueous solution and in-situ photopolymerisation (J. Phys.: Condens. Matter, 16, (2004), 4137-4144). Studies regarding formation and stability of LC colloids depending on surfactant have been performed by Spillmann et al. (Langmuir, 25, (2009), 2419-2426), in which 200 nm colloids were prepared by sonication of the RM dissolved in chloroform and water. Terentjev et al. synthesized particles made of main chain liquid crystal polymers via mini-emulsion technique (Nature Materials, 4, (2005), 486-490). They observed ellipsoidal particles with different aspect ratio depending on the particle size.
In the described prior art, the molecular orientation of the liquid crystal inside the particle was not determined because the particles were too small to be resolved by optical microscopy.
Zentel et al. showed that RM particles could also be made via dispersion polymerization in mixtures of ethanol/methoxyethanol (Macromolecules, 39, (2006), 8326-8333) and THF/silicon oil (Macromol. Chem. Phys, 210, (2009), 1394-1401 and Soft Matter, (2010), 6, 4112-4119). The spherical particles obtained by this method show optical anisotropy and could be manipulated under electric fields and optical tweezers. Zentel et al also synthesized elastomeric LC particles via microfluidics showing ellipsoidal shape (Adv. Mater, (2009), 21, 4859-4862). However, all these particles were only synthesised using monoacrylate RMs, which limits the range of application and flexibility of their properties. Micron sized spherical colloids synthesised by Zentel et al. exhibit optical anisotropy. However, a mixture of different director orientations is present inside the same system and they do not show shape anisotropy.
The above literature describes syntheses of particles which are not suitable for scale-up of the particles, i.e. using microfluidics it is only possible to produce a small volume of particles per day. The particles are made from a very limited range of RMs and so do not have desirable properties. In prior art, synthesis of particles has only been described using polar solvents and not low dielectric solvent. For display applications, in particular in EPD, it is crucial not to introduce polar impurities which will have an adverse effect on the operation of the display. It is known that if particles are prepared in a polar solvent, it is very difficult to remove all traces of the solvent. Traces of polar solvent on particle surfaces affect the ability of the particles to be transferred into a final solvent used in a display. In addition, particles which are prepared in a polar solvent and transferred to a non-polar solvent suitable for EPD such as dodecane or hexadecane will not disperse well and give poor display performance. It is therefore desirable to prepare particles for display applications, in particular for EPD in a non-polar solvent, and preferably in a solvent such as dodecane which can be used directly in the final display application; thus avoiding solvent transfer which is both expensive, time costly and can introduce undesired impurities in the display.